1. Field of the Invention
The present invention relates to a molded part such as a pipe part with its through hole outsert-molded with resin and a mold used for outsert molding.
2. Description of the Prior Art
Recently, molded products have been increasingly produced, which are formed by outsert-molding a resin into a through hole of a pipe-like part.
In case resin is molded on the outer surface of a metal work (insert-molding), a stress acts in a contraction direction of the resin, and there are no adverse effects between the resin and an insert part, thus allowing desired molding. In case resin is molded on the inner surface of the metal work (outsert molding), however, since a resin contracts after molding, various adverse effects are found especially when high-precision parts are to be formed.
Outsert molding in a conventional pipe part will be described in detail below.
FIGS. 1A and 1B are sectional views of a molded part formed by outsert molding in a pipe-like part. FIG. 1A shows a state of the molded part immediately after molding. FIG. 1B shows a state of the molded part when a predetermined period of time passed after molding.
FIG. 1A shows a molded part 1 in its final form. More specifically, a metal pipe part 2 has a through hole 3 for molding a resin 5, and shoulder portions 4 at both ends of the through hole 3. The resin 5 is molded into a shape matching the through hole 3 and collar portions, which define an axial hole 7, in accordance with the shape of the pipe part 2. As shown in FIG. 1A, immediately after molding, the resin 5 flows and is formed into a shape perfectly matching the through hole 3 and the collar portions 6. However, when a given period of time elapses, the resin 5 contracts to form gaps (I)8 and (II)9 between the pipe part 2 and the resin 5. The finished molded part 1 has looseness.
A conventional mold for forming the molded part 1 will be described below with reference to FIGS. 2 to 4.
FIG. 2 is a sectional view of a conventional mold. FIG. 3 is a plan view of a fixing slide jig. FIG. 4 is a plan view of a fixing slide jig for multiple molding.
Reference numeral 10 denotes a mold constituted by a movable half mold 11 and a stationary half mold 12.
A fixing slide jig 13 having an insertion port 14 is fitted/held in the movable half mold 11. The fixing slide jig 13 serves to fix the pipe part 2 (shown in FIGS. 1A and 1B) used for outsert mold. Two inclined loose fitting holes 15 are obliquely formed in the fixing slide jig 13 so as to be symmetrical, and inclined surfaces 20 are formed on the two ends of the fixing slide jig 13. A pin 16 and a knockout pin 17 are formed on the movable half mold 11 so as to be vertically slidable. The pin 16 is used to form the axial hole 7 of the molded part 1. The knockout pin 17 is used to take out the molded product 1 from the movable half mold 11.
The stationary half mold 12 is designed to be vertical movable so as to take out the molded part 1. An open pin 18 and a positioning block 21 are fixed to the stationary die 12. The open pin 18 is engaged with the inclined loose fitting hole 15 upon upward movement of the stationary die 12 so as to laterally open the stationary slide jig 13 on a cut plane 19 as a boundary. The positioning block 21 serves to regulate the separation position of the stationary slide jig 13. Reference numeral 22 denotes a gate for injecting a resin.
An operation of the mold will be described below.
When the pipe part 2 is inserted into the insertion hole 14 and the movable half mold 11 is moved upward in FIG. 2, the open pins 18 are fitted in the inclined loose fitting holes 15, and the inclined surface 20 is brought into contact with the positioning block 21 to close the fixing slide jig 13, thus holding the pipe part 2. A resin is then injected through the gate 22 to perform outsert molding. Upon completion of outsert molding, the movable half mold 11 is moved downward. As a result, the open pins 18 are disengaged from the inclined loose fitting holes 15, and the inclined surface 20 is moved away from to the positioning block 21 to open the fixing slide jig 13 on the cut plane 19 as a boundary to form surfaces 19a and 19b. The knockout pin 17 protrudes upward to extract the molded part 1. When the pipe part 2 is inserted again, and the movable half mold 11 is lowered, the fixing slide jig 13 is closed, and the resin is injected, thereby completing the molded part 1.
FIG. 4 shows a fixing slide jig for obtaining a plurality of finished parts 1 by using five insertion holes 14a to 14e. Similar to the operation described with reference to FIG. 3, a fixing slide jig 23 is laterally opened on a cut plane 25. Pipe parts 2 are inserted into the insertion holes 14a to 14e. The fixing slide jig 23 is then closed, and a resin is injected therein, thus simultaneously forming the five molded parts 1 by molding.
In the case of the molded part shown in FIG. 1A, the molded part formed by outsert molding has a proper shape immediately after molding as shown in FIG. 1A, but the resin contracts after a lapse of a predetermine period of time. As a result, as shown in FIG. 1B, although no gaps are formed in the longitudinal direction because it coincides with a clamping direction, gaps I and II are formed between the piper part 2 and the resin 5 in the radial direction upon contraction of the resin. This impairs eccentricity precision of the outer diameter of the molded product 1 with respect to the axial hole 7.
With a further lapse of time, the eccentricity precision of the outer diameter with respect to the axial hole is further degraded due to a creep phenomenon. For this reason, a deviation of .+-.4 .mu.m, of an outer diameter with respect to an axial hole, which is a precision required for rollers, cannot be expected.
With regard to a mold, when a mold for a single molded part is used, fairly high precision can be expected in outsert molding. However, when a mold for multiple molded parts is to be used to reduce the cost, which may has the structure shown in FIG. 4, since the eccentricity of a cavity is larger than that of a cavity located inward therefrom and the fixing slide jig is laterally moved, an outer diameter deviation of a molded part becomes larger than that of a molded part located inward therefrom. Therefore, it is very difficult to perform molding with desired precision.